A thermostat is simply a switch that turns on or off a furnace/air conditioner. A 'programmable thermostat' changes the parameters of the switching. Everyone understands that a furnace will run less if the temperature is set lower, obviously reducing the amount of fuel used to maintain the desired climate. There are 4 types of programmable thermostats; 5-2, 7-day, adaptive, and evolution style. 

1. 5-2 refers to how the days are allocated for programming. The 5-2 is the most basic of the programmables. The days are are lumped together for programming Mon-Fri and Saturday/Sunday. The programming for a basic 5-2 allows for 4 change points throughout the day: wake, leave, return and bed. Most folks have regular schedules through the week allowing for uniform changes of temperature. A sample day might be Mom waking up at 5:30 to start everyone on their day. So for the "wake" setting we would set the time at 5:15 and the temperature to 71 degrees. By 7:45 everyone is dressed fed and out the door so we could put the "leave" setting at 7:30 and 60 degrees. The kids begin getting home at 4:00 to the "return" setting set at 66 degrees. And finally bed time at 10:00, with the "bed" setting at 9:30 and 60 degrees. Saturday and Sunday will generally have a "wake" and "bed" setting. Don't worry about catching the occasional chill...all programmables that I've seen have an override function that allows change of temperature until the next scheduled program. Theses units are available at most home centers for about $30 and are very easy to change.

2. 7-day stats allow individual programming by day not week. These are great for anyone with a varying schedule in that it allows more control of the programming. Other than that, it works just like the 5-2.
Just plan on more time in programming and spending about $75. Installation will be the same as a 5-2.

3. Adaptive recovery. These are installed and programmed the same as the 7-day. The difference is that the adaptive thermostat will "learn" your house and HVAC system. How? Simple. With the 5-2, if you wake up at 5:30 and the stat changes at 5:30 your house will be 60 degrees. The adaptive unit will start the furnace in the middle of the night and raise your house temp one degree so it "learns" how long it will take to reach your "wake" setting. At 5:30 when you roll out of bed the house is where you want it. This is really nice during the hottest days of summer. Instead of coming home to a hot house with a struggling A/C you arrive to your desired temperature.
Plan on spending about $125 for one.

4. Evolution style. Evolution is the name Bryant gives its top of the line system. These are whole house controllers running temperature, humidity, and indoor air quality equipment. Carrier and Honeywell both make similar systems. These units are installed with all new equipment by dealers and are unavailable to homeowners at this time.


5. Savings. Estimating savings earned with programmables is difficult. I have heard as much as 3% per degree below 70 degrees, which I find unlikely. A total of 3-4% is much easier to believe. For about a $100  and an hours time, even at  the low end of the estimation programmable thermostats will have a quick payback.